Spiral antenna devices are used in a myriad of applications requiring broad frequency coverage. These devices typically include patch or microstrip antennas having Archimedean, logarithmic, equiangular, sinuous or multi-arm planar configurations, as described in U.S. Pat. No. 5,508,710, issued Apr. 16, 1996 to Wang et al. In general, antenna elements are disposed on dielectric substrates, which radiate outwardly from both sides of the substrate.
A cavity is placed on one side of the substrate to trap or absorb radiation in an unwanted direction. The trapped radiation or energy must be either terminated or recombined with radiation in a desired direction, so that a resulting radiation pattern is not adversely affected.
A cavity having a depth of a quarter wavelength (λ/4) may combine two wavefronts in phase. The ability to combine these wavefronts is dependent on the relative phase between the direct and the reflected components of the wavefronts. Since combining these wavefronts is frequency dependent, the antenna device results in a narrow-band device. In practice, the cavity is absorber-loaded to mask the reflective cavity back wall and eliminate unwanted signals from interfering with a desired radiation pattern. Under these circumstances, the spiral antenna device dissipates half the signal and is primarily used in receivers, which are limited to low power.
U.S. Pat. No. 5,815,122 (issued on Sep. 29, 1998 to Nurnberger et al.), U.S. Pat. No. 5,589,842 (issued on Dec. 31, 1996 to Wang et al.), and U.S. Pat. No. 6,407,721 (issued on Jun. 18, 2002 to Mehen et al) disclose ways of eliminating the λ/4 cavity depth in an attempt to provide thin conformal radiators. Eliminating the λ/4 cavity depth is of particular is interest at UHF/VHF frequencies, where cavity depths are measured in feet and are impractical for deployment on airborne platforms. Cavity depths of one hundredth of a wavelength (λ/100) are disclosed to achieve thin conformal devices. These devices, however, are limited to low power receiving applications.
In addition to dissipating at least half the power, another limitation on the power capacity of a spiral antenna is its RF feed network, known as a balun. The balun is a component providing excitation to the spiral antenna. The balun is typically placed in transmission lines carrying low power and, if placed in a cavity that is not highly absorptive, generates multiple cavity resonances.
A need exists for an antenna that is suitable not only for broadband receiving functions, but also suitable for transmitting functions that can sustain high peak and average power. A need also exists for alternate means of feeding antenna terminals that eliminates the balun, because the balun is power limited.
A need further exists for a spiral antenna suitable for use in broadband phased arrays, and suitable in modular construction of such phased arrays.
Yet another need exists for a spiral antenna device whose depth is small, permits conformal installation, and when used in high power applications includes efficient cooling means.
This invention addresses these needs.